The integrated provirus of MH2 has been molecularly cloned. Its 5.2-kb genome has the genetic structure 5'-Deltagag(2.0 kb)-mht(1.3 kb)-myc(1.3 kb)-c(0.4 kb)-poly(A)-(0.2 kb)-3'. myc is the onc-specific sequence shared by the MC29 subgroup, including MC29, CMII, and OK10 viruses, while mht is a cell-derived, MH2-specific sequence. The nucleotide sequence of 3.5 kb from the 3' end of Deltagag to the 3' end of cloned, proviral, MH2 DNA has been determined. The following results were obtained: (i) Deltagag-mht forms a hybrid gene with a contiguous reading frame of 2682 nucleotides that terminates with a stop codon near the 3' end of mht. The 969 nucleotides of mht, including the stop codon, are 80% sequence-related to the onc-specific raf sequence of MSV 3611 (94% homologous at the deduced amino acid level). (ii) The myc sequence is preceded by an RNA splice acceptor site shared with the cellular proto-myc gene, beyond which it is colinear up to a 3' termination codon and 40 noncoding nucleotides with the myc sequences of MC29 and chicken proto-myc. It seems that myc forms, together with a 5' exon derived from the partial gag gene, a second MH2-specific gene. It is concluded MH2 contains two genes with oncogenic potential: the Deltagag-mht gene, which is closely related to the Deltagag-raf transforming gene of MSV 3611, and the myc gene, which is closely related to the transforming gene of MC29. Our studies indicate that the molecularly cloned MH2 DNA transforms chicken embryo fibroblasts on transfection with helper DNA, or on superinfection with helper virus. A transforming virus was recovered from the supernatant of these transformed cells. T1 oligonucleotide analysis of the RNA extracted from this virus identified the virus to be MH2. Experiments using deletion mutants in the Deltagag, mht, and myc genes are in progress in order to elucidate the oncogenic functions of these genes.